1. Technical Field
The present invention relates generally to electronic pens used with an electrostatic digitizing tablet and in particular to battery powered electronic pens. Still more particularly, the present invention relates to an energy efficient high voltage oscillator for battery powered electronic pens used with an electrostatic digitizing tablet.
2. Description of the Related Art
Electrostatic digitizer tablets provide an indication of the location of an electronic pen on a planar sensor grid. The electronic pen (sometimes called a stylus) transmits an electric field by emitting a radio frequency (RF) signal which may be modulated. An oscillator within the pen provides the RF signal.
The electronic pen may be physically coupled to the tablet through wiring (generally referred to as a xe2x80x9ctetheredxe2x80x9d pen) which provides power to the pen. Alternatively, the pen may be battery powered and unconnected to the tablet (xe2x80x9cuntetheredxe2x80x9d). Untethered pens provide a more natural user interface and are therefore preferable.
Within a battery powered electronic pen, the oscillator consumes the most power. The oscillator is typically a high output voltage oscillator, generated output voltages in the range of 80-100 Vp-p (volts peak-to-peak). Since the pen is powered by a small, watch-type battery, an extremely energy efficient oscillator is desirable. On the other hand, the operating frequency of the oscillator should be well-defined and stable. Satisfying both requirements for an electronic pen oscillator design is quite challenging. Conventional electronic tuning methods may not be employed because the high voltage amplitude prevents the use of available tuning components such as varicaps or varactors.
Given a power supply limited to 3-5 V, a high Q tank circuit at the output of the oscillator is the most energy efficient mechanism for obtaining the high output voltage required. However, a tank circuit generally yields an operating frequency tolerance of 5-10%, which may be unacceptable for many applications. Automatic electronic tuning of the tank circuitxe2x80x94with a phase lock loop (PLL), for examplexe2x80x94is virtually impossible for circuit oscillation amplitudes in the range of 80-100 Vp-p.
Additionally, the high voltage pen output signal usually requires additional modulation to transmit pen status information. Such information, called telemetry, may typically include tip switch status, barrel switch status, battery status, etc. Efficient output signal modulation of a high Q tank circuit with a voltage amplitude swing of 100 Vp-p at the oscillator output is difficult.
It would be desirable, therefore, to provide an energy efficient oscillator having a very stable and very well defined operating frequency which produces an output voltage in the range of at least 80 Vp-p. It would further be desirable to provide an oscillator output circuit which permits efficient phase shift keyed (PSK) or frequency shift keyed (FSK) modulation of a high voltage output signal.
A high Q tank circuit is employed at the output of a digital, crystal controlled oscillator to generate a high voltage amplitude signal. The tank circuit has a resonant frequency greater than the maximum required oscillation frequency. During each oscillation cycle, oscillation within the tank circuit is stopped in an energy efficient manner such that the resonant oscillation period is extended to match the required oscillation period. Modulation of the digital oscillator signal appears in the output circuit signal.